The invention relates to a driving hub for a vehicle, particularly a bicycle, with a friction gear of which the transmission ratio is infinitely adjustable.
Known from the manual entitled "Bicycling Science", Frank Rowland Witt, David Gordon Wilson, The MIT Press, Cambridge, London, 1988, 2nd edition, page 282 are a number of principles concerning the construction of friction gear mechanisms, attention being drawn to the problems of such friction gears when used in conjunction with bicycle hubs. These problems reside in the fact that with bicycle hubs, relatively high torque levels have to be transmitted at low rotary speeds, it being intended that the transmission ratio be so adjustable that, according to requirements, so the hub sleeve will rotate more rapidly or more slowly than the driver. One of the types of construction shown in principle is a friction gear in the form of a ball friction gear, where friction balls rigidly mounted on angularly adjustable ball spindles are able to roll on the conical outer periphery of both a driving wheel and also of a driven wheel in order to transmit a corresponding torque from the driving pulley to the driven pulley.
DE-21 36 243 A1 discloses a stationary gear mechanism with an input shaft and an output shaft coaxial with it. The friction balls used here are mounted to be freely rotatable and roll on a total of four bearing surfaces which are adjustable in relation to one another, the axis of rotation of the balls adjusting itself accordingly. The drive of the friction balls is effected via an inclined outer peripheral surface of a driving pulley which is connected to the driving axle. An expanding coupling is provided to increase the applied pressure as the torque increases. The construction of this gear mechanism is complicated. A direct and precise adjustment of a desired transmission ratio is not possible. The fixed casing in which there is inter alia a drive worm of the control arrangement cannot be replaced by a rotating hub sleeve.
The U.S. Pat. No. 4,735,430 discloses a driving hub for bicycles with a friction gear based on the principle of torque transmission by means of friction wheels which roll on toroidal surfaces of mutually opposite toroidal discs. In contrast to conventional driving hubs, in this case the hub axle is rotatably mounted in the bicycle frame so that special installation means are required. Also, a planetary gear mechanism is necessarily provided in order to drive the hub sleeve in the direction of rotation of the driver; however, this impairs the efficiency of the driving hub and increases the structural cost as well as the hub weight.
An improvement to this hub is disclosed by EP 0 432 742 A1. As is conventional with bicycles, the hub axle can be rigidly installed within the bicycle frame with, for friction gear control, a linkage which extends through the hollow hub axle. Also this bicycle hub works on the principle of the toroidal discs. In order to adjust the rotary speed range of this hub to the transmission ratios which are normally found in bicycles, an additional planetary gear mechanism is required which in turn impairs the efficiency of the driving hub and increases the structural cost as well as the hub weight.